1. Field of the Invention
The present invention relates to a controller for controlling a spindle of a machine tool that is driven by an induction motor via a belt by controlling the rotational velocity of the induction motor. More particularly, the present invention relates to a controller of a spindle which is adapted to be capable of controlling the spindle with an encoder attached to the spindle for detecting the position of the spindle and without a velocity detector attached to the induction motor.
2. Description of the Related Art
FIG. 15 is a schematic block diagram illustrating a spindle to which an induction motor having no velocity detection sensor attached thereto (sensorless) is applied.
A sensorless induction motor 2 transmits its driving force to a spindle 6 of a lathe or the like via a reduction mechanism 4 including a belt. An encoder 8 that outputs a feedback pulse Pfb for detecting the rotational position of the spindle 6 is attached to the spindle 6. The feedback pulse Pfb output from the encoder 8 attached to the spindle 6 is used for control of the rotational position of the spindle 6 to achieve a function such as a function of halting the spindle at a fixed position.
In a subtractor 10, the velocity of the sensorless induction motor 2 estimated by a velocity estimator 15 is subtracted from a velocity command Vcmd output from a host controller (not shown) to determine velocity deviation. A velocity control unit 11 performs PI control (proportional-plus-integral control) on the velocity deviation to determine a torque current command. A current control unit 12 determines a voltage command by using the determined torque current command and an actual current Ire flowing through the induction motor 2 detected by a current detector 14, and outputs the determined voltage command to a power amplifier 13 comprising an inverter. The velocity estimator 15 estimates the rotational velocity of the induction motor 2 by using the actual current Ire flowing through the induction motor 2 detected by the current detector 14 to determine an estimated velocity Vest.
FIG. 16 is a schematic block diagram illustrating a spindle to which the sensorless induction motor is applied and for which the feedback pulse from the encoder is utilized also for motor velocity control.
The feedback pulse Pfb output from the encoder 8 is used not only for control of the rotational position of the spindle 6 but also for calculation of the estimated velocity in the velocity estimator 15.
FIG. 17 is a schematic block diagram illustrating a spindle to which the sensorless induction motor is applied and for which the feedback pulse from the encoder is utilized also for current control.
The feedback pulse output from the encoder 8 is used not only for control of the rotational position of the spindle 6 but also for calculation of the voltage command in the current control unit 12.
In motor control of a general sensorless induction motor 2, as described above referring to FIGS. 15 to 17, the velocity of the motor is determined by estimated calculation performed in software based on the actual current Ire flowing through the induction motor 2, and calculation of the torque command and determination of current phases to be applied to each winding of the motor are performed based on this estimated velocity Vest.
When this sensorless motor is applied to a spindle of a lathe as a machine tool, the encoder 8 for detecting the rotational position of the spindle is mounted on the spindle. In this regard, Japanese Patent Application Laid-Open No. 2002-51594 discloses a technique to achieve higher controllability for simple sensorless control by substituting the feedback pulse Pfb of the encoder 8 for motor velocity feedback or by using the feedback pulse Pfb of the encoder 8 for correction on the estimated velocity Vest of the induction motor 2, for correction of an excitation frequency command or for processing such as clamp.
Generally, a machine tool such as a lathe drives its spindle by means of the induction motor 2 via a reduction mechanism including a belt between the induction motor 2 and the spindle, and slip of the belt may occur when sudden and large cutting force acts. Since the position of the spindle is detected by the encoder 8 attached to the spindle, the slip does not affect the accuracy of the positioning of the spindle, if any.
However, when the velocity (rotational velocity) of the encoder attached to the spindle is used for motor velocity control (see FIG. 16) or current control (see FIG. 17), the torque command may be suddenly changed to generate large shock and prevent the current phase control from being performed correctly, activating an alarm for warning of overcurrent (see FIG. 19). This is because the motor velocity converted from the encoder velocity suddenly changes in the case of the slip as illustrated in FIGS. 18A and 18B.